A quantitative study was undertaken of the anisotropy of low-strain mechanical behavior for specially oriented polyethylene with controlled crystalline and lamellar orientation. The samples were prepared by the die drawing of injection-molded rods of polyethylene and annealing. This produced a parallel lamellar structure for which a simple, three-dimensional composite laminate model could be used to calculate the expected anisotropy. Experimental data, including X-ray strain measurements of the lateral crystalline elastic constants, showed good quantitative agreement with the model prediction. The X-ray strain measurements confirmed that the amorphous regions exert large constraints on the crystalline phase in the lateral directions, where an order of magnitude difference was found between the measured apparent lateral crystalline compliances in the lamellar-stack sample and the expected values for a perfect crystal.